System and method for position determination by impulse radio using round trip time-of-flight

ABSTRACT

A system and a method for position determination by impulse radio using a first transceiver having a first clock providing a first reference signal and a second transceiver placed spaced from the first transceiver. The system determines the position of the second transceiver. The second transceiver has a second clock that provides a second reference signal. A first sequence of pulses are transmitted from the first transceiver. The first sequence of pulses are then received at the second transceiver and the second transceiver is then synchronized with the first sequence of pulses. A second sequence of pulses are transmitted from the second transceiver. The first transceiver receives the second sequence of pulses and the first transceiver is synchronized with the second sequence of pulses. A delayed first reference signal is generated in response to the synchronization with the second sequence of pulses. A time difference between the delayed first reference signal and the first reference signal is then measured. The time difference indicates a total time of flight of the first and second sequence of pulses. The distance between the first and the second transceiver is determined from the time difference. The direction of the second transceiver from the first transceiver is determined using a directional antenna. Finally, the position of the second transceiver is determined using the distance and the direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/517,161, filed Apr. 5,2000 (now U.S. Pat. No. 6,297,773, issued Oct. 2, 2001), which is adivisional of Application Ser. No. 09/045,929, filed Mar. 23, 1998 (nowU.S. Pat. No. 6,133,876, issued Oct. 17, 2000).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to position determination, and morespecifically to a system and method for position determination byimpulse radio.

2. Background Art

In recent years, modem communications technology has provided varioussystems for position determination. The global positioning system (GPS)operated by the United States Department of Defense, for example, is ahighly complex system of determining the position of an object. The GPSsystem depends on measuring the time-of-flight of microwave signals fromthree or more orbiting satellite transmitters by a navigation receiverthat computes the position of the mobile unit. According to the GPSsystem, each satellite broadcasts a time-stamped signal that includesthe satellite's ephemeris, i.e., its own position. When the mobile unitreceives a GPS signal, the mobile unit measures the transmission delayrelative to its own clock and determines the pseudo-range to thetransmitting satellite's position. The GPS system requires threesatellites for two-dimensional positioning, and a fourth satellite forthree-dimensional positioning.

Another approach is that employed by the U.S. Navy's TRANSIT system. Inthat system, a mobile unit performs continuous doppler measurements of asignal broadcast by a low earth orbit (LEO) satellite. The measurementscontinue for several minutes. The system usually requires two passes ofthe satellite, necessitating a wait of more than 100 minutes. Inaddition, because the position calculations are performed by the mobileunit, the satellite must broadcast information regarding its position,i.e., its ephemeris. Although the TRANSIT system is capable of highaccuracy (on the order of one meter), the delay required is unacceptablefor commercial applications.

Although these systems accurately determine the unknown position of anobject, they are extremely complex, and, more importantly, expensive toimplement. For example, both the GPS and TRANSIT systems requiremultiple satellites, sophisticated receivers and antennas that requirehundreds of millions dollars of investments. Also, response times of GPSand TRANSIT systems are typically slow due to their narrow bandwidth.Furthermore, since these systems depend on orbiting satellites, theyrequire an unimpeded view of the sky to operate effectively.

There is a great need in many different fields for a simple, lessexpensive alternative to complicated position determination systems. Onesuch area is a typical shipping terminal, e.g., a major sea-port or anairport. In a sea-port, containers having valuable cargo are stored atwarehouses or are left in designated places in the terminals. Also,containers are sometimes moved from one section of the port to anothersection in preparation for their eventual loading into a cargo ship orbeing picked up by trucks or railcars after being unloaded from a cargoship. Often it is necessary to determine the location of one or morecontainers. However, it is difficult to identify one or more containersamong hundreds, or thousands of containers in a terminal. Similarproblems are also encountered in airports and railway terminals wherecontainers are kept in storage sites.

A simple, less expensive position determination system is also desirablefor locating police units. Such a position determination system can beused as a vehicle locator system. A city dispatcher would be able toquickly and efficiently dispatch police units if the dispatcher haspre-existing knowledge of each unit's locations. Currently citydispatchers use mobile phones to communicate with police units in orderto know their locations. However, using mobile phones to determine thepositions of the police units has some disadvantages. Use of mobilephones is expensive and time consuming. Also, when a police officer isnot in the car, it is not possible to determine the unit's location.

Recently, the FCC has mandated that all cell phone systems implementposition determination for use in emergency call location. In addition,there is a need for position determination as part of cell phonesecurity, fraudulent use, and zone hand off algorithms. Theserequirements are difficult to meet and GPS is not adequate to reliablydeliver the required accuracy.

For these reasons, it is clear that there is a need for a simple, lowcost position determination system.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a system and a method for positiondetermination using impulse radios. According to one embodiment of thepresent invention, a first transceiver having a first clock providing afirst reference signal is positioned. A second transceiver whoseposition is to be determined is spatially displaced from the firsttransceiver. The second transceiver has a second clock that provides asecond reference signal.

To determine the position of the second transceiver, a first sequence ofpulses are transmitted from the first transceiver. The first sequence ofpulses are then received at the second transceiver and the secondtransceiver is then synchronized with the first sequence of pulses.Then, a second sequence of pulses are transmitted from the secondtransceiver. The first transceiver receives the second sequence ofpulses and the first transceiver is synchronized with the secondsequence of pulses. A delayed first reference signal is generated inresponse to the synchronization with the second sequence of pulses.Then, a time difference between the delayed first reference signal andthe first reference signal is measured. The time difference indicates atotal time of flight of the first and second sequence of pulses.

Then, the distance between the first and the second transceiver isdetermined from the time difference. Then, the direction of the secondtransceiver from the first transceiver is determined using a directionalantenna. Finally, the position of the second transceiver is determinedusing the distance and the direction.

In another embodiment of the present invention a plurality of firsttransceivers and a second transceiver are placed such that eachtransceiver is spaced from the others. The distance between each firsttransceiver and the second transceiver is measured. Then, the positionof the second transceiver is determined using a triangulation method.

In yet another embodiment of the present invention, the secondtransceiver is placed in a mobile telephone whose position is to bedetermined. This allows a user of a mobile telephone to determine his orher exact location.

The position determination system according to the present inventionprovides numerous advantages over conventional position determinationsystems described before. For example, the present invention does notrequire the use of expensive orbiting satellites. Thus, the presentinvention is less expensive to implement. Also, signals from orbitingsatellites are often impeded by obstacles, such as trees or overheadstructures. Since, the present invention does not require the use oforbiting satellites, the operation of the present invention is notimpeded by obstacles, such as trees or other structures. Furthermore,since the present invention utilizes ultra-wideband signals, it providesa relatively fast response time. As a result, the position of an objectcan be determined much faster than it would be possible using existingsystems.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the-drawing in which the reference numberfirst appears.

FIGS. 1A and 1B show a 2 GHz center frequency monocycle pulse in thetime and frequency domains, respectively, in accordance with the presentinvention.

FIGS. 2A and 2B are illustrations of a 1 mpps system with 1 ns pulses inthe time and frequency domains, respectively, in accordance with thepresent invention.

FIG. 3 illustrates a modulating signal that changes the pulse repetitioninterval (PRI) in proportion to the modulation in accordance with thepresent invention.

FIG. 4 is a plot illustrating the impact of pseudo-random dither onenergy distribution in the frequency domain in accordance with thepresent invention.

FIG. 5 illustrates the result of a narrowband sinusoidal (interference)signal overlaying an impulse radio signal in accordance with the presentinvention.

FIGS. 6A and 6B show received pulses at a cross correlator and outputsignal at the cross correlator, respectively.

FIGS. 7A and 7B illustrate impulse radio multipath effects in accordancewith the present invention.

FIG. 8 illustrates the phase of the multipath pulse in accordance withthe present invention.

FIG. 9 illustrates one embodiment of an impulse radio transmitteraccording the present invention.

FIG. 10 illustrates one embodiment of an impulse radio receiveraccording to the present invention.

FIG. 11 illustrates one embodiment of the present invention comprisingtwo impulse radios and a direction finding antenna.

FIGS. 12A and 12B are timing diagrams illustrating the operation of theembodiment of FIG. 11.

FIG. 13 shows another embodiment of the present invention comprisingthree impulse radios.

FIG. 14 is an operational flow diagram illustrating the steps involvedin FIG. 13.

FIG. 15 illustrates a phenomenon known as position ambiguity.

FIG. 16 illustrates yet another embodiment of the present invention thatresolves the position ambiguity of FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

Overview of the Invention

The present invention is directed to a system and a method for positiondetermination using impulse radios. Impulse radio was first fullydescribed in a series of patents, including U.S. Pat. Nos. 4,641,317(issued Feb. 3, 1987), 4,813,057 (issued Mar. 14, 1989), 4,979,186(issued Dec. 18, 1990) and 5,363,108 (issued Nov. 8, 1994) to Larry W.Fullerton. A second generation of impulse radio patents include U.S.Pat. Nos. 5,677,927 (issued Oct. 14, 1997), 5,687,169 (issued Nov. 11,1997) and co-pending application Ser. No. 08/761,602 (filed Dec. 6,1996) to Fullerton et al. These patent documents are incorporated hereinby reference.

Prior to a detailed description of the present invention, a high levelexplanation of the invention is provided. According to one embodiment ofthe present invention, a first transceiver having a first clockproviding a first reference signal is positioned. A second transceiverwhose position is to be determined is placed spaced from the firsttransceiver. The second transceiver has a second clock that provides asecond reference signal.

A first sequence of pulses are transmitted from the first transceiver.The first sequence of pulses are then received at the second transceiverand the second transceiver is then synchronized with the first sequenceof pulses. Next, a second sequence of pulses are transmitted from thesecond transceiver. The first transceiver receives the second sequenceof pulses and the first transceiver is synchronized with the secondsequence of pulses. Next, a delayed first reference signal is generatedin response to the synchronization with the second sequence of pulses.Next, a time difference between the delayed first reference signal andthe first reference signal is measured. The time difference indicates atotal time of flight of the first and second sequence of pulses.

Next, the distance between the first and the second transceiver isdetermined from the time difference. Then, the direction of the secondtransceiver from the first transceiver is determined using a directionfinding antenna. Finally, the position of the second transceiver isdetermined using the distance and the direction.

In another embodiment of the present invention, the second transceiveris placed in a mobile telephone whose position is to be determined. Thisallows a mobile telephone network to determine a user's exact location.Additional embodiments are described in detail below in the sectiontitled “Position Determination by Impulse Radio.”

Impulse Radio Basics

Impulse radio refers to a radio system based on a waveform that isessentially the impulse response of the available bandwidth. An idealimpulse radio waveform is a short Gaussian monocycle. As the namesuggests, this waveform attempts to approach one cycle of RF energy at adesired center frequency. Due to implementation and other spectrallimitations, this waveform may be altered significantly in practice fora given application. Most waveforms with enough bandwidth approximate aGaussian shape to a useful degree.

Impulse radio can use many types of modulation, including AM, time shift(also referred to as pulse position) and Mary versions. The time shiftmethod has simplicity and power output advantages that make itdesirable. In this document, the time shift method is used as anillustrative example.

In impulse radio communications, the pulse-to-pulse interval is variedon a pulse-by-pulse basis by two components: an information componentand a pseudo-random code component. Generally, spread spectrum systemsmake use of pseudo-random codes to spread the normally narrow bandinformation signal over a relatively wide band of frequencies. A spreadspectrum receiver correlates these signals to retrieve the originalinformation signal. Unlike direct sequence spread spectrum systems, thepseudo-random code for impulse radio communications is not necessary forenergy spreading because the monocycle pulses themselves have aninherently wide bandwidth. Instead, the pseudo-random code is used forchannelization, energy smoothing in the frequency domain, jammingresistance and reducing the signature of a signal to an interceptreceiver.

The impulse radio receiver is typically a homodyne receiver with a crosscorrelator front end in which the front end coherently converts anelectromagnetic pulse train of monocycle pulses to a baseband signal ina single stage. The baseband signal is the basic information channel forthe basic impulse radio communications system, and is also referred toas the information bandwidth. It is often found desirable to include asubcarrier with the base signal to help reduce the effects of amplifierdrift and low frequency noise. The subcarrier that is typicallyimplemented alternately reverses modulation according to a known patternat a rate faster than the data rate. This pattern is reversed again justbefore detection to restore the original data pattern. This methodpermits AC coupling of stages, or equivalent signal processing toeliminate DC drift and errors from the detection process. This method isdescribed in detail in U.S. Pat. No. 5,677,927 to Fullerton et al.

The data rate of the impulse radio transmission is only a fraction ofthe periodic timing signal used as a time base. Each data bit typicallytime position modulates many pulses of the periodic timing signal. Thisyields a modulated, coded timing signal that comprises a train ofidentically shaped pulses for each single data bit. The cross correlatorof the impulse radio receiver integrates multiple pulses to recover thetransmitted information.

Waveform

Impulse radio refers to a radio system based on a waveform thatapproaches the impulse response of the available bandwidth. In thewidest bandwidth embodiment, the resulting waveform approaches one cycleper pulse at the center frequency. In more narrow band embodiments, eachpulse consists of a burst of cycles usually with some spectral shapingto control the bandwidth to meet desired properties such as out of bandemissions or in-band spectral flatness, or time domain peak power orburst off time attenuation.

In the course of system analysis and design, it is convenient to modelthe desired waveform in an ideal sense to provide insight into theoptimum behavior for detail design guidance. One such waveform modelthat has been useful is the Gaussian monocycle as shown in FIG. 1A. Thiswaveform is representative of the transmitted pulse produced by a stepfunction into an ultra-wideband antenna. The basic equation normalizedto a peak value of 1 is as follows:${f_{mono}(t)} = {\sqrt{e}\quad \left( \frac{t}{\sigma} \right)\quad ^{\frac{- t^{2}}{2\quad \sigma^{2}}}}$

where σ is a time scaling parameter,

t is time,

ƒ_(mono)(t) is the waveform voltage, and

e is the natural logarithm base.

The frequency domain spectrum of the above waveform is shown in FIG. 1B.The corresponding equation is:${F_{mono}(f)} = {\left( {2\pi} \right)^{\frac{2}{3}}\quad \sigma \quad f\quad ^{{- 2}\quad {({\pi \quad \sigma \quad f})}^{2}}}$

The center frequency (ƒ_(c)) or frequency of peak spectral density is:$f_{c} = \frac{1}{2\quad {\pi\sigma}}$

These pulses, or burst of cycles, may be produced by methods describedin the patents referenced above or by other methods that are known toone of ordinary skill in the art. Any practical implementation willdeviate from the ideal mathematical model by some amount, which may beconsiderable since impulse radio systems can tolerate seeminglyconsiderable deviation with acceptable system consequences. This isespecially true in the microwave implementations where precise waveformshaping is difficult to achieve.

These mathematical models are provided as an aid to describing the idealoperation and are not intended to limit the invention. In fact, anyburst of cycles that adequately fills a given bandwidth and has anadequate on-off attenuation ratio for a given application will serve thepurpose of this invention.

One of the great advantages of measuring distances and locatingpositions using this waveform is that the pulse is short enough forindividual cycles to be identified so that ambiguity is removed anddistance can be resolved to better than a cycle given adequate signal tonoise ratio. This can be done by locking onto the signal at incrementalcycle points and noting which one has the greatest amplitude. This lockpoint will be the main lock point and can be used to calibrate thesystem.

A Pulse Train

Although one or more bit per pulse systems have been conceived, impulseradio systems typically use pulse trains, not single pulses, forcommunications. As described in detail below, the impulse radiotransmitter produces and outputs a train of pulses for each bit ofinformation.

Prototypes built by the inventors have pulse repetition frequencies ofbetween 0.7 and 10 meg a pulse per second (mpps, where each megapulse is10⁶ pulses). FIGS. 2A and 2B are illustrations of a 1 mpps system with(uncoded, unmodulated) 1 nanosecond (ns) pulses in the time andfrequency domains (see 102 and 104, respectively). In the frequencydomain, this highly regular pulse train produces energy spikes (comblines 204) at one megahertz intervals; thus, the already low power isspread among the comb lines 204. This pulse train carries no informationand, because of the regularity of the energy spikes, might interferewith conventional radio systems at short ranges.

Impulse radio systems typically have very low duty cycles so the averagepower in time domain is significantly lower than its peak power in thetime domain. In the example in FIGS. 2A and 2B, the impulse transmitteroperates 0.1% of the time (i.e., 1 ns per microsecond (μs)).

Additional processing is needed to modulate the pulse train so that theimpulse radio system can actually communicate information. Theadditional processing also smooths the energy distribution in thefrequency domain so that impulse radio transmissions (e.g., signals)interfere minimally with conventional radio systems.

Modulation

Any aspect of the waveform can be modulated to convey information.Amplitude modulation, phase modulation, frequency modulation, time shiftmodulation and M-ary versions of these have been proposed. Both analogand digital forms have been implemented. Of these, digital time shiftmodulation has been demonstrated to have various advantages and can beeasily implemented using a correlation receiver architecture.

Digital time shift modulation can be implemented by shifting the codedtime position by an additional amount δ. With this method, themodulation shift is very small relative to the code shift. In a 10 mppssystem with a center frequency of 2 GHz, for example, the coded pulseposition may be anywhere within 100 ns, but any given pulse would bespecified to be at its assigned position within 30 picoseconds (ps). Themodulation would deviate this position by 75 ps, early or late, torepresent a 1 or a 0 at that level of coding. Note that this istypically not the final data level of coding, but a pseudo Manchestersubcarrier level of coding.

Thus, a train of n pulses is each delayed a different amount from itsrespective time base clock position by a code delay amount plus amodulation amount, where n is the number of pulses associated with agiven data symbol digital bit.

Coding for Energy Smoothing and Channelization

Because the receiver is a cross correlator, the amount of time positionmodulation required for one-hundred percent modulation is calculated by1/(4f_(c)) (where f_(c) is the center frequency). For a monocycle with acenter frequency of 2.0 GHz, for example, this corresponds to ±125 (ps)of time position modulation. The spectrum-smoothing effects at thislevel of time dither is negligible.

Impulse radio achieves optimal smoothing by applying to each pulse apseudo-random noise (PN) code dither with a much larger magnitude thanthe modulation dither. FIG. 4 is a plot illustrating the impact of PNcode dither on energy distribution in the frequency domain. FIG. 4, whencompared to FIG. 2B, shows the impact of using a 256 position PN coderelative to an uncoded signal.

PN code dithering also provides for multi-user channelization(channelization is a technique employed to divide a communications pathinto a number of channels). In an uncoded system, differentiatingbetween separate transmitters would be very hard. The PN codes createchannels, if the PN codes themselves are relatively orthogonal (i.e.,there is low correlation and/or interference between the codes beingused).

Reception and Demodulation

Clearly, if there were a large number of impulse radio users within aconfined area, there might be mutual interference. Further, while the PNcoding minimizes that interference, as the number of users rises, theprobability of an individual pulse from one user's sequence beingreceived simultaneously with a pulse from another user's sequenceincreases. Fortunately, implementations of an impulse radio according tothe present invention do not depend on receiving every pulse. Theimpulse radio receiver performs a correlating, synchronous receivingfunction (at the RF level) that uses a statistical sampling of manypulses to recover the transmitted information.

Impulse radio receivers typically integrate 100 or more pulses to yieldthe demodulated output. The optimal number of pulses over which thereceiver integrates is dependent on a number of variables, includingpulse rate, bit rate, jamming levels, and range.

Jam Resistance

Besides channelization and energy smoothing, the PN coding also makesimpulse radio highly resistant to jamming from all radio communicationssystems, including other impulse radio transmitters. This is critical asany other signals within the band occupied by an impulse signal act as ajammer to the impulse radio. Since there are no unallocated bandsavailable for impulse systems, they must share spectrum with otherconventional radios without being adversely affected. The PN code helpsimpulse systems discriminate between the intended impulse transmissionand interfering transmissions from others.

FIG. 5 illustrates the result of a narrowband sinusoidal jamming(interference) signal 502 overlaying an impulse radio signal 504. At theimpulse radio receiver, the input to the cross correlator would includethat narrowband signal 502, as well as the received ultrawide-bandimpulse radio signal 504. Without PN coding, the cross correlator wouldsample the jamming signal 502 with such regularity that the jammingsignals could cause significant interference to the impulse radioreceiver. However, when the transmitted impulse signal is encoded withthe PN code dither (and the impulse radio receiver is synchronized withthat identical PN code dither) it samples the jamming signals randomly.According to the present invention, integrating over many pulses negatesthe impact of jamming. In statistical terms, the pseudo-randomization intime of the receive process creates a stream of randomly distributedvalues with a mean of zero (for jamming signals).

Processing Gain

Impulse radio is jam resistant because of its large processing gain. Fortypical spread spectrum systems, the definition of processing gain,which quantifies the decrease in channel interference when wide-bandcommunications are used, is the ratio of the bandwidth of the channel tothe bandwidth of the information signal. For example, a direct sequencespread spectrum system with a 10 kHz information bandwidth and a 16 MHzchannel bandwidth yields a processing gain of 1600 or 32 dB. However,far greater processing gains are achieved with impulse radio systemswhere for the same 10 KHz information bandwidth and a 2 GHz channelbandwidth the processing gain is 200,000 or 53 dB.

The duty cycle (e.g., of 0.5%) yields a process gain of 23 dB. (Theprocess gain is generally the ratio of the bandwidth of a receivedsignal to the bandwidth of the received information signal.) Theeffective over sampling from integrating multiple pulses to recover theinformation (e.g., integrating 200 pulses) yields a process gain of 23dB. Thus, a 2 GHz divided by a 10 mpps link transmitting 50 kilobits persecond (kbps) would have a process gain of 46 dB, (i.e., 0.5 ns pulsewidth divided by a 100 ns pulse repetition interval would have a 0.5%duty cycle, and 10 mps divided by a 50,000 bps would have 200 pulses perbit.)

Capacity

Theoretical analyses suggest that impulse radio systems can havethousands of voice channels. To understand the capacity of an impulseradio system one must carefully examine the performance of the crosscorrelator. FIG. 6B shows the “cross correlator transfer function” 602.This represents the output value of an impulse radio receiver crosscorrelator as a function of received pulse timing. As illustrated at604, the cross correlator's output is zero volts when pulses arriveoutside of a cross correlation window 606. As pulse arrival time variesalong the time axis of FIG. 6A, the corresponding correlator outputintegral varies according to FIG. 6B. It is at its maximum (e.g., 1volt) when the pulse is τ/4 ahead of the center of the window (as shownat 610), zero volts when centered in the window (as shown at 612); andat its minimum (e.g., −1 volt) when it is τ/4 after the center.

When the system is synchronized with the intended transmitter, the crosscorrelator's output has a swing of maximum value, e.g., between ±1 volt(as a function of the transmitter's modulation). Other in-bandtransmission would cause a variance to the cross correlator's outputvalue. This variance is a random variable and can be modeled as aGaussian white noise signal with a mean value of zero. As the number ofinterferers increases the variance increases linearly. By integratingover a large number of pulses, the receiver develops an estimate of thetransmitted signal's modulation value. Thus, the: $\begin{matrix}{{{Variance}\quad {of}\quad {the}\quad {Estimate}} = \frac{N\quad \sigma^{2}}{\sqrt{Z}}} & (1)\end{matrix}$

Where N=number of interferers,

σ² is the variance of all the interferers to a single cross correlation,and

Z is the number of pulses over which the receiver integrates to recoverthe modulation.

This is a good relationship for a communications system for as thenumber of simultaneous users increases, the link quality degradesgradually (rather than suddenly).

Multipath and Propagation

Multipath fading, the bane of sinusoidal systems, is much less of aproblem (i.e., orders of magnitude less) for impulse systems than forconventional radio systems. In fact, Rayleigh fading, so noticeable incellular communications, is a continuous wave phenomenon, not an impulsecommunications phenomenon.

In an impulse radio system in order for there to be multipath effects,special conditions must persist. The path length traveled by thescattered pulse must be less than the pulse's width times the speed oflight, and/or successively emitted pulses at the transmitter (in thesequence) arrive at the receiver at the same time.

For the former with a one nanosecond pulse, that equals 0.3 meters orabout 1 foot (i.e., 1 ns×30.0,000,000 meters/second). (See FIG. 7, inthe case where the pulse traveling “Path 1” arrives one half a pulsewidth after the direct path pulse.)

For the latter with a 1 megapulse per second system that would be equalto traveling an extra 300, 600 or 900 meters. However, because eachindividual pulse is subject to the pseudo-random dither, these pulsesare decorrelated.

Pulses traveling between these intervals do not cause self-interference(in FIG. 7, this is illustrated by the pulse traveling Path 2). Whilepulses traveling grazing paths, as illustrated in FIG. 7 by thenarrowest ellipsoid, create impulse radio multipath effects.

As illustrated in FIG. 8 at 802, if the multipath pulse travels one halfwidth of a pulse width further, it increases the power level of thereceived signal (the phase of the multipath pulse will be inverted bythe reflecting surface). If the pulse travels less than one half a pulsewidth further it will create destructive interference, as shown at 804.For a 1 ns pulse, for example, destructive interference will occur ifthe multipath pulse travels between about 0 and 15 cm (0 and 6 inches).

Position Determination By Impulse Radio

Although, the advantages of the impulse radio technology have beendemonstrated in voice and data communication, an additional area thatcan benefit from the impulse radio technology is position determination.The impulse radio technology can be advantageously utilized to determinethe position of an object, and it can provide a less expensive, simpleralternative to the GPS and the TRANSIT systems discussed earlier.

The present invention is a system and a method for positiondetermination by impulse radio technology. The preferred embodiments ofthe invention are discussed in detail below. While specific steps,configurations and arrangements are discussed, it should be understoodthat this is done for illustrative purposes only. A person skilled inthe relevant art will recognize that other steps, configurations andarrangements can be used without departing from the spirit and scope ofthe present invention.

FIG. 9 illustrates an embodiment of an impulse radio transmitter 900according to the present invention that can be used in positiondetermination. Referring now to FIG. 9, transmitter comprises a timebase 904 that generates a periodic timing signal 908. The time base 904comprises a voltage controlled oscillator, or the like, which istypically locked to a crystal reference, having a high timing accuracy.The periodic timing signal 908 is supplied to a code source 912 and acode time modulator 916.

The code source 912 comprises a shift register, a computational deviceor a storage device such as a random access memory (RAM), read onlymemory (ROM), or the like, for storing codes and outputting the codes ascode signal 920. In one embodiment of the present invention, orthogonalPN codes are stored in the code source 912. The code source 912 monitorsthe periodic timing signal 908 to permit the code signal to besynchronized to the code time modulator 916. The code time modulator 916uses the code signal 920 to modulate the periodic timing signal 908 forchannelization and smoothing of the final emitted signal. The output ofthe code time modulator 916 is called a coded timing signal 924.

The coded timing signal 924 is provided to an output stage 928 that usesthe coded timing signal as a trigger to generate pulses. The pulses aresent to a transmit antenna 932 via a transmission line 936 coupledthereto. The pulses are converted into propagating electromagnetic wavesby the transmit antenna 932. The electromagnetic waves propagate to animpulse radio receiver (shown in FIG. 10) through a propagation medium,such as air.

FIG. 10 illustrates an impulse radio receiver 1000 according to oneembodiment of the present invention that can be used in positiondetermination. Referring now to FIG. 10, the impulse radio receiver 1000comprises a receive antenna 1004 for receiving a propagatingelectromagnetic wave and converting it to an electrical signal, referredherein as the received signal 1008. The received signal is provided to across correlator 1016 via a transmission line 1012 coupled to thereceive antenna 1004.

The receiver 1000 comprises a decode source 1020 and an adjustable timebase 1024. The decode source 1020 generates a decode signal 1028corresponding to the code used by the associated transmitter 900 thattransmitted the propagated signal. The adjustable time base 1024generates aperiodic timing signal 1032 that comprises a train oftemplate signal pulses having waveforms substantially equivalent to eachpulse of the received signal 1008.

The decode signal 1028 and the periodic timing signal 1032 are receivedby the decode timing modulator 1036. The decode timing modulator 1036uses the decode signal 1028 to position in time the periodic timingsignal 1032 to generate a decode control signal 1040. The decode controlsignal 1040 is thus matched in time to the known code of the transmitter900 so that the received signal 1008 can be detected in the crosscorrelator 1016.

The output 1044 of the cross correlator 1016 results from the crossmultiplication of the input pulse 1008 and the signal 1040 and theintegration of the resulting signal. This is the correlation process.The signal 1044 is filtered by a low pass filter 1048 and a signal 1052is generated at the output of the low pass filter 1048. The signal 1052is used to control the adjustable time base 1024 to lock onto thereceived signal. The signal 1052 corresponds to the average value of thecross correlator output, and is the lock loop error signal that is usedto control the adjustable time base 1024 to maintain a stable lock onthe signal. If the received pulse train is slightly early, the output ofthe low pass filter 1048 will be slightly high and generate a time basecorrection to shift the adjustable time base slightly earlier to matchthe incoming pulse train. In this way, the receiver is held in stablerelationship with the incoming pulse train. Further impulse radioreceiver and transmitter embodiments are described in the 5,677,927 and5,687,169 patents noted above.

FIGS. 11-16 illustrate system level diagrams of several embodiments ofthe present invention using one or more impulse radios.

FIG. 11 illustrates the present invention in its simplified form,wherein first and second impulse radios 1104 and 1108 and a directionfinding antenna 1112 are used to determine the position of an object O.

The impulse radios 1104 and 1108 are each configured to provide thefunctionalities of both a transmitter and a receiver. The first impulseradio 1104 and the direction finding antenna are at a location (x1, y1).The second impulse radio 1108 is mounted on the object O whose position(x2, y2) is to be determined. The object O is located at a distance dfrom the first impulse radio 1104. Note that with all of the embodimentsof this invention where the receiver or the transmitter is mounted onthe object O or a reference point, it is not necessary to mount theantenna 1112 at such point.

FIGS. 12A and 12B are timing diagrams illustrating the operation of theembodiment of FIG. 11. For the sake of simplicity, the operation of thepresent invention is illustrated using a reference clock pulse (FIG.12A) in conjunction with pulse trains (FIG. 12B). In actual operation, asequence of reference clock pulses are generated by clocks at theimpulse radios 1104 and 1108. The reference clock pulses are thenprocessed by the impulse radios and are used to generate to pulse trainsshown in FIG. 12B. The shape of the actual transmitted waveform is shownin FIG. 2A.

Referring now to FIG. 12A, a reference clock pulse 1204 is generated bythe impulse radio 1104 at time t₁. The reference clock pulse 1204corresponds to the transmission of a pulse train 1220 by the impulseradio 1104. (Also at time t₁, a pulse train 1220 is transmitted by theimpulse radio 1104.) The pulse train 1220 is received by the impulseradio 1108 at time t₂, at which time the impulse radio 1108 synchronizesits own clock with the pulse train 1220. A reference clock pulse 1208generated by the impulse radio 1108 corresponds to the synchronizationof the impulse radio 1108 with the pulse train 1220.

Next, at time t₃, the impulse radio 1108 transmits a pulse train 1230. Areference clock pulse 1212 generated by the impulse radio 1108 at timet₃ corresponds to the transmission of the pulse train 1230. Thus, t₃-t₂is the elapsed time between when the impulse radio 1108 receives thepulse train 1220 and the time the impulse radio 1108 transmits the pulsetrain 1230. The pulse train 1230 is received by the impulse radio 1104at time t₄ at which time the impulse radio 1104 synchronizes itself withthe pulse train 1230. A reference clock pulse 1216 generated by theimpulse radio 1104 at time t₄ corresponds to the impulse radio 1104synchronizing itself with the pulse train 1230.

Next, the time difference between the reference clock pulse 1216 and thereference clock pulse 1204 is determined. The time difference is givenby t₄-t₁. The time difference represents the elapsed time between thetransmission of the pulse train 1220 by the impulse radio 1104 and thereception of the pulse train 1230 by the impulse radio 1104. The time offlight is given by (t₄-t₁)-(t₃-t₂), where (t₃-t₂) is the delayencountered at the impulse radio 1108. The time (t₃-t₂) can be resolvedby a system calibration step where the transceivers are set up at knowndistances and an estimated time representing (t₃-t₂) is used tocalculate distance. Any error becomes a correction factor to besubtracted from all subsequent distance measurements, or alternativelythe estimated time representing (t₃-t₂) can be updated to show thecorrect distance and the updated time used for subsequent distancecalculations.

Next, the distance d is calculated from the time of flight. Then, theangular direction φ of the impulse radio 1108 is determined by thedirection finding antenna 1112. The angular direction φ of the impulseradio 1108 is the angle of the impulse radio 1108 with respect to thefirst impulse radio 1104. Finally, the position (x2, y2) of the object Ois determined using the distance d and the angular direction φ.

In another embodiment of the present invention, further simplificationand cost reduction is achieved by using a passive receiver method.According to the passive receiver method, the impulse radio 1104 isconfigured solely as a receiver, while the impulse radio 1108 isconfigured solely as a transmitter. The impulse radios 1104 and 1108 aresynchronized by a universal clock, i.e., an external clock or an atomicclock. In other words, internal clocks (or voltage controlledoscillators (VCOs)) of the impulse radios 1104 and 1108 are in sync withan external clock, i.e., a universal clock. This insures that theinternal clocks (or VCOs) of the impulse radio run synchronously. Thesynchronization can be achieved by initializing clocks prior to theimpulse radios being deployed into operation. The details of suchsynchronization would be apparent to a person skilled in the relevantart.

In operation, at time t, impulse radios 1104 and 1108 each generate areference clock pulse T1. Also, at time t₁, the impulse radio 1108transmits a sequence of pulses (S₁). S₁ is received by the impulse radio1104. The impulse radio 1104 then synchronizes itself with S₁ andproduces a delayed reference clock pulse T1′. The impulse radio 1104then determines the time difference (T1′-T1). The impulse radio 1104then calculates the position (x2, y2) according to the techniquedescribed above.

In yet another embodiment of the present invention, a third impulseradio can be substituted in lieu of the direction finding antenna forposition determination. FIG. 13 shows an embodiment of the presentinvention having three impulse radios 1304, 1308 and 1312. The first andthe second impulse radio 1304 and 1308 are located at positions (x1, y1)and (x2, y2), respectively, each spaced from the other by a distance d,.The third impulse radio 1312 is mounted on the object O whose position(x3, y3) is to be determined. The object O is located at distances d₂and d₃ from the first and the second impulse radio 1304 and 1308,respectively.

FIG. 14 is an operational flow diagram illustrating the method ofdetermining the position of the object O in accordance with theembodiment of FIG. 13. In a step 1404, the time of flight (also referredto as the first time of flight) between the first impulse radio 1304 andthe third impulse radio 1312 is determined. In a step 1408, the time offlight (also referred to as the second time of flight) between thesecond impulse radio 1308 and the third impulse radio 1312 isdetermined. In a step 1412, the distance d₂ is determined from the firsttime of flight. In a step 1416, the distance d₃ is determined from thesecond time of flight. Finally, in a step 1420, the position (x3, y3) ofthe object O is calculated from d₂, d₃, (x1, y1) and (x2, y2) using atriangulation method. The distance d₁ can be measured and provides acheck on the relative coordinates (x1, y1) and (x2, y2). Thisinformation and any error can be used to update the measurement system.

Again, further simplification and cost reduction of the embodiment ofFIG. 13 can be achieved by using a passive receiver method. According tothe passive receiver method, the first and the second impulse radio 1304and 1308 are each configured solely as a receiver, while the thirdimpulse radio 1312 is configured solely as a transmitter. The first,second and third impulse radios 1304, 1308 and 1312 are synchronized bya universal clock. The synchronization of the clocks can be achieved byinitializing the clocks prior to the impulse radios being deployed intooperation. Other synchronization techniques can be employed as would beapparent to a person skilled in the relevant art. In operation, thedistances d₂ and d₃ are measured using methods described earlier. Then,the position of the object (x3, y3) is determined from d₁, d₂ and d₃ bya triangulation method.

The use of only three impulse radios results in a phenomenon known asposition ambiguity, which is illustrated in FIG. 15. Briefly stated,position ambiguity refers to the condition wherein a triangulationmethod provides two solutions for the position of the object. Onesolution is the actual position (x3, y3) of the object, while the othersolution (x3′, y3′) is a mirror image of the actual position. Referringnow to FIG. 15, a triangulation method provides a solution thatindicates that the object may be located at either (x3, y3) or at (x3′,y3′). This ambiguity is resolved by the use of a direction findingantenna placed at or near the first or the second impulse radio 1504 or1508. The direction finding antenna can be utilized to accuratelyascertain the true position of the object by determining the angulardirection φ of the object O. An alternative method is to position two ormore directional antennas such that their respective coverage areas eachfavor different position ambiguity areas. These antennas may bealternately selected and the relative signal strength used to determinewhich antenna is receiving the stronger signal. This would thus resolvethe position ambiguity. The directional antennas may be electrically ormechanically steered array antennas. The details of ascertaining thetrue position of the object by a directional antenna are beyond thescope of the present invention and would be apparent to a person skilledin the relevant art.

In the alternative, a fourth impulse radio can be used to resolve theposition ambiguity, and this is shown in FIG. 16. Referring now to FIG.16, first, second and third impulse radios 1604, 1608 and 1612 areplaced at locations (x1, y1), (x2, y2) and (x3, y3), respectively. Afourth impulse radio 1616 is mounted on the object whose position (x4,y4) is to be determined. The object O is at a distance d₃, d₄ and d₅from the first, second and third impulse radios 1604, 1608, 1612,respectively. The distance d, between the first and second impulseradios and the distance d₂ between the second and the third impulseradios are known. Using methods described previously, the distances d₃,d₄ and d₁ are determined. Then, the position (x4, y4) of the object O iscalculated by any known triangulation methods.

Another phenomenon known as elevation ambiguity may exist if the impulseradios of FIG. 16 are not coplanar. The elevation ambiguity can beresolved by using a fifth impulse radio.

Recently, the mobile telephone industry has received a mandate from theFederal Communication Commission (FCC) to install position determinationsystems in mobile telephones. According to the FCC mandate, a mobiletelephone network must be able to locate a caller of an emergency 911call within 30 meters of accuracy. Although various technologies toimplement this feature are currently being considered, no singletechnology has emerged as feasible.

The position determination system according to the present invention canbe conveniently used to meet the FCC mandate. According to oneembodiment of the present invention, an impulse radio can be used tolocate the position of a mobile telephone user.

According to yet another embodiment of the present invention, a mobiletelephone is equipped with an impulse radio receiver. The impulse radioreceiver locks onto three beacons (or train of pulses), wherein eachbeacon is being transmitted by a base station. Thus, the mobiletelephone simultaneously communicates with three base stations (three ormore transmitted beacons are required to resolve the position ambiguityof the mobile phone). This can be performed by equipping the mobilephone with three separate cross correlators or a fast cross correlator.Other methods that are well known to a person skilled in the art can beemployed to lock onto three separate beacons. Then, the time of flightof each beacon is computed by the mobile telephone. Then, using themethods described above, the position of the mobile phone is computed.Finally, the mobile telephone transmits the information to the basestations.

Several other variations of the above embodiment can be implemented. Forexample, a mobile telephone can be equipped with an impulse radiotransmitter. The transmitter can transmit three beacons to three basestations (i.e. each base station receives a beacon). Each base stationcomputes the distance between the mobile telephone and the base stationfrom the time of flight of the respective beacon. The base stations thentransmit the information regarding the measured distances to one of thebase stations selected from the three base stations. The selected basestation then computes the position of the mobile telephone using themeasured distances.

In yet another embodiment of the present invention, digital data,digitized voice, and/or analog modulation may be transmitted on the datachannel while positioning is independently derived from timinginformation. The transmitter and the receiver used in this embodiment isdescribed in detail in U.S. Pat. No. 5,677,927 noted above.

The present invention can also be used in a GPS system to provide forgreater accuracy. In fact, using the present invention, the GPS systemcould be updated, or another system could be deployed to deliver greateraccuracy.

The principle limitation of the GPS system is that there is noconvenient way to match carrier cycles with modulation cycles, making itvery difficult to combine the coarse resolution available frommodulation with the fine resolution available from carrier phase. Thus,designers are left with a choice between absolute range and resolutionbased on modulated information that is accurate within 5 meters usingfull military capability, and relative range based on carrier phase thatis accurate within a few centimeters, but the system must start at aknown point.

With a GPS system employing impulse radio transmitters and receivers, itis possible to determine the time domain equivalent of carrier phase toabsolute accuracy, to thereby resolve subcycle time differences thatpermit range accuracy and resolution within a few centimeters. Thisleaves propagation effects as the largest remaining error source, sincetime errors and other implementation effects can generally be reduced toacceptable levels.

The claimed invention provides several solutions to problems faced bydesigners of position determination systems. In the past, it was notobvious to the designers how to use pulses in a practical positiondetermination system. The problem is that it is difficult to generate asingle pulse of adequate power to propagate over a useful range. Thedetection of a single pulse is also difficult and requires large asignal to noise ratio. The claimed invention avoids this problem byusing pulse trains. With pulse trains, it is possible to add the energyfrom many pulses to achieve the equivalent effect of one single pulse.In the claimed invention, time position coding of pulse trains is usedso that the repeat length of a coded pulse train is longer than thedistance to be measured, thus resolving a potential range ambiguityresulting from a rapid pulse rate.

In the claimed invention, the time difference between the transmittedand received pulse trains is measured indirectly by measuring the phasedifference between the associated corresponding time bases that are usedto generate the pulse trains. The effect of an entire pulse train isaveraged in the above described loop lock filter so that pulse timingerrors are reduced, while signal to noise ratio is improved byintegration gain. The integration method in the cross correlator isfully described in the above noted patents. This requires extremelystable and accurate time bases. In one embodiment of the claimedinvention, the time bases are generated from high frequency clockstypically phase locked to a crystal reference. The high frequency clocksare counted down using a binary counter with a modulo count equal to themodulo repeat length of the pulse position code, which is used toprevent range ambiguities from the repetitive pulse trains.

When the impulse radio position determination system is operated in anarea of high multipath and/or the line of sight between the transmitterand receiver is blocked, the largest signal that the receiver mayreceive may not represent the shortest distance between the receiver andthe transmitter. This will result in an error in the estimate ofdistance between the transmitter and the receiver. Specifically, thedistance will be over estimated. One way to resolve this would be toallow the receiver to lock onto the largest available signal, whether areflection or not, and then search for earlier signals with longer dwelltimes and narrower information bandwidths in order to find the earliestsignal. In the case of urban positioning, the earliest signal may not bediscernable. However, if there are a plurality of either beacons orremote receivers scattered over the area of interest, the uncertaintymay be reduced by statistical methods such as finding the centroid ofthe area bounded by the range estimates, or the least squares of thedata method or other techniques that are known to those skilled in theart.

Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. Thus the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. An impulse radio system, comprising: a firstimpulse radio transceiver configured to transmit a first ultra widebandsignal; a second impulse radio transceiver spaced apart from the firstimpulse radio transceiver and not time synchronized with the firstimpulse radio transceiver, the second impulse radio transceiver beingconfigured to receive the first ultra wideband signal, the secondimpulse radio being configured to transmit a second ultra widebandsignal responsive to the first ultra wideband signal; and wherein thefirst impulse radio transceiver is configured to receive the secondultra wideband signal, the first impulse radio transceiver beingconfigured to determine an elapsed time between the transmission of thefirst ultra wideband signal and the reception of the second ultrawideband signal indicative of a total time-of-flight of the first andsecond ultra wideband signals.
 2. The system of claim 1, wherein: thefirst ultra wideband signal includes a first pulse train having ultrawideband frequency domain characteristics including a center frequencyin the GigaHertz (GHz) range and a frequency bandwidth in the GHz range;and the second ultra wideband signal includes a second pulse trainhaving ultra wideband frequency domain characteristics including acenter frequency in the GHz range and a frequency bandwidth in the GHzrange.
 3. The system of claim 1, wherein: the first ultra widebandsignal includes a first pulse train comprising a first sequence ofpulses spaced apart in time; and the second ultra wideband signalincludes a second pulse train comprising a second sequence of pulsesspaced apart in time.
 4. The system of claim 3, wherein each of thefirst and second sequences of pulses has a pulse repetition frequency ofbetween 0.7 and 10 megapulses (10⁶) per second.
 5. The system of claim3, wherein at least some of the pulses in each of the first and secondsequences of pulses have pulse widths on the order of one nanosecond. 6.The system of claim 1, wherein the first impulse radio transceiverdetermines a distance between the first and second impulse radiotransceivers based at least in part on the elapsed time.
 7. The systemof claim 6, wherein the distance is determined based on a timedifference between (i) the elapsed time, and (ii) a delay representing atime difference between the reception of the first ultra wideband signalat the second impulse radio transceiver and the transmission of thesecond ultra wideband signal from the second impulse radio transceiver,wherein the time difference between the elapsed time and the delayrepresents the total time-of-flight of the first and second ultrawideband signals.
 8. The system of claim 7, wherein a correction factoris used to determine the distance.
 9. The system of claim 7, wherein thedelay is an estimated delay.
 10. The system of claim 9, wherein acorrection factor is used to update the delay.
 11. The system of claim6, further comprising: a direction finding antenna, coupled to the firstimpulse radio transceiver, for determining an angular direction (φ) ofthe second impulse radio transceiver relative to the first impulse radiotransceiver, wherein the first impulse radio transceiver determines aposition of the second transceiver based on the distance and the angulardirection (φ).
 12. The system of claim 1, wherein the first impulseradio transceiver includes a first clock configured to generate a firstclock pulse corresponding to the transmission of the first ultrawideband signal, and a second clock pulse corresponding to the receptionof the second ultra wideband signal, and wherein the first impulsetransceiver is configured to determine the elapsed time based on thefirst and second clock pulses.
 13. The system of claim 12, wherein thesecond impulse radio transceiver includes a second clock configured togenerate a third clock pulse corresponding to the reception of the firstultra wideband signal, and a fourth clock pulse corresponding to thetransmission of the second ultra wideband signal.
 14. The system ofclaim 1, wherein at least one of the first and second impulse radiotransceivers is combined with a Global Positioning System (GPS)receiver.
 15. The system of claim 1, wherein the second transceiver isinstalled in a mobile telephone.
 16. The system of claim 1, wherein atleast one of the first and second ultra wideband signals includes one ofamplitude modulation, phase modulation, frequency modulation and timeshift modulation.
 17. The system of claim 1, wherein at least one ofdigital data, digitized voice, and analog information is communicatedbetween the first and second impulse radio transceivers via at least oneof the first and second ultra wideband signals.
 18. An impulse radiosystem, comprising: a first impulse radio transceiver including a clock,the clock configured to generate a first clock pulse, the first impulseradio being configured to transmit a first ultra wideband signalresponsive to the first clock pulse; a second impulse radio transceiverspaced apart from the first impulse radio transceiver and configured toreceive the first ultra wideband signal, the second impulse receiverbeing configured to transmit a second ultra wideband impulse signalresponsive to the first ultra wideband signal; and wherein the firstimpulse radio transceiver is configured to receive the second ultrawideband signal, the clock being configured to generate a second clockpulse responsive to the second ultra wideband impulse signal, the firstimpulse transceiver being configured to determine an elapsed timebetween the first and second clock pulses indicative of a total time offlight of the first and second ultra wideband signals.
 19. The system ofclaim 18, wherein the first and second impulse radio transceivers arenot time synchronized with each other.
 20. In an impulse radio systemincluding a first impulse radio transceiver and a second impulse radiotransceiver spaced from the first impulse radio transceiver, a methodcomprising: (a) transmitting from the first impulse radio transceiver afirst ultra wideband signal; (b) receiving the first ultra widebandsignal at the second impulse radio transceiver; (c) transmitting, fromthe second impulse radio transceiver, a second ultra wideband signalresponsive to the first ultra wideband signal; (d) receiving the secondultra wideband signal at the first impulse radio transceiver; and (e)determining an elapsed time, between the transmission of the first ultrawideband signal and the reception of the second ultra wideband signal,indicative of a total time-of-flight of the first and second ultrawideband signals.
 21. The method of claim 20, wherein: the first ultrawideband signal includes a first pulse train having ultra widebandfrequency domain characteristics including a center frequency in theGigaHertz (GHz) range and a frequency bandwidth in the GHz range; andthe second ultra wideband signal includes a second pulse train havingultra wideband frequency domain characteristics including a centerfrequency in the GHz range and a frequency bandwidth in the GHz range.22. The method of claim 20, wherein: the first ultra wideband signalincludes a first pulse train comprising a first sequence of pulsesspaced apart in time; and the second ultra wideband signal includes asecond pulse train comprising a second sequence of pulses spaced apartin time.
 23. The method of claim 22, wherein each of the first andsecond sequences of pulses has a pulse repetition frequency of between0.7 and 10 megapulses (10⁶) per second.
 24. The method of claim 22,wherein at least some of the pulses in each of the first and secondsequences of pulses have pulse widths on the order of one nanosecond.25. The method of claim 20, further comprising: (f) determining adistance between the first and second impulse radio transceivers basedat least in part on the elapsed time determined in step (e).
 26. Themethod of claim 25, wherein step (f) comprises: determining the distancebetween the first and second impulse radio transceivers based on a timedifference between (i) the elapsed time, and (ii) a delay representing atime difference between the reception of the first ultra wideband signalat the second impulse radio transceiver and the transmission of thesecond ultra wideband signal from the second impulse radio transceiver,wherein the time difference between the elapsed time and the delayrepresents the total time-of-flight of the first and second ultrawideband signals.
 27. The method of claim 26, wherein a correctionfactor is used to determine the distance.
 28. The method of claim 26,further comprising, prior to step (f): producing an initial estimate ofthe delay; and adjusting the initial estimate of the delay with acorrection factor.
 29. The method of claim 25, further comprising: (g)determining an angular direction (φ) of the second impulse radiotransceiver relative to the first impulse radio transceiver; and (h)determining a position of the second transceiver based on the distanceand the angular direction (φ).
 30. The method of claim 20, furthercomprising: generating at the first impulse radio transceiver a firstclock pulse corresponding to the transmission of the first ultrawideband signal; and generating at the first impulse radio transceiver asecond clock pulse corresponding to the reception of the second ultrawideband signal, wherein step (e) comprises determining the elapsed timebased on the first and second clock pulses.
 31. The method of claim 30,further comprising: generating at the second impulse radio transceiver athird clock pulse corresponding to the reception of the first ultrawideband signal; and generating at the second impulse radio transceivera fourth clock pulse corresponding to the transmission of the secondultra wideband signal.
 32. In an impulse radio system including a firstimpulse radio transceiver and a second impulse radio transceiver spaceda known distance from the first impulse radio transceiver, a method forcalibrating distance measurements, comprising: (a) transmitting from thefirst impulse radio transceiver a first ultra wideband signal; (b)receiving the first ultra wideband signal at the second impulse radiotransceiver; (c) transmitting, from the second impulse radiotransceiver, a second ultra wideband signal in response to the firstultra wideband signal; (d) receiving the second ultra wideband signal atthe first impulse radio transceiver; and (e) determining an elapsed timerepresenting a time difference between the transmission of the firstultra wideband signal and the reception of the second ultra widebandsignal; (e) producing an estimated delay representing a time differencebetween the reception of the first ultra wideband signal and thetransmission of the second ultra wideband signal; (f) determining anestimated distance between the first and second impulse radio based onthe elapsed time and the estimated delay; and (g) performing one ofdetermining an error distance between the estimated distance and theknown distance, and updating the estimated delay based on the knowndistance.
 33. An impulse radio system, comprising: a first impulse radiotransceiver configured to transmit a first ultra wideband signal; asecond impulse radio transceiver spaced apart from the first impulseradio transceiver and not time synchronized with the first impulse radiotransceiver, the second impulse radio transceiver being configured toreceive the first ultra wideband signal, the second impulse radio beingconfigured to transmit a second ultra wideband signal responsive to thefirst ultra wideband signal after a delay; and wherein the first impulseradio transceiver is configured to receive the second ultra widebandsignal, the first impulse radio transceiver being configured todetermine an elapsed time between the transmission of the first ultrawideband signal and the reception of the second ultra wideband signal,the first impulse radio transceiver being configured to determine a timedifference between the elapsed time and the delay indicative of a totaltime-of-flight of the first and second ultra wideband signals.
 34. In animpulse radio system including a first impulse radio transceiver and asecond impulse radio transceiver spaced from the first impulse radiotransceiver, a method comprising: (a) transmitting from the firstimpulse radio transceiver a first ultra wideband signal; (b) receivingthe first ultra wideband signal at the second impulse radio transceiver;(c) transmitting, from the second impulse radio transceiver, a secondultra wideband signal; (d) receiving the second ultra wideband signal atthe first impulse radio transceiver; (e) determining an elapsed timebetween the transmission of the first ultra wideband signal and thereception of the second ultra wideband signal; (f) determining a delaybetween the reception of the first ultra wideband signal and thetransmission of the second ultra wideband signal; and (g) determining adifference between the elapsed time and the delay indicative of thetotal time-of-flight of the first and second ultra wideband signals. 35.The method of claim 34, further comprising: (h) determining a distancebetween the first and second impulse radio transceivers based on thetime-of-flight.